TKANSACTIONS OP THE SECTIONS. 79 



The Dufton Shales are shown to constitute a mass of fossiliferous shales imder- 

 Ijiiig the Coniston Limestone, without the intervention of any igneous rock of con- 

 temporaneous oiigin. They pass upwards into the limestone, and are about 

 300 feet thick under the Pennme chain, but thin out in proceeding westwards. 

 The Coiiiston Limestone is the only constant member of its series, and the Trimicleus 

 Shales form a local group developed above it in the Sedbergh district. The entire 

 Coniston Limestmie series is shown to be identical with the limestones of Kildare 

 and Pomeroy in Ireland and Girvau in Scotland. 



The Coniston Mtidstone series is divided into two distinct groups, the Skelgill 

 Beds and the Knock Beds. The name of Skelgill Beds i& given by the authors to a 

 group of black graptolitic mudstones and shales, which seem rarely or never to 

 exceed GO feet in thickness, and which are found almost eveiywhere above the 

 Coniston Limestone. These beds are replete with Graptolites belon^ng to an ex- 

 ceedingly well-marked group of forms, which can be precisely paralleled with the 

 species" characteristic of the highest beds of the Iloffut series of the south of Scot- 

 land {Birkhill Beds). Similar Graptolites occur in certain black beds which occupy 

 a similar position on the shores of Belfast Lough and elsewhere in Ireland. 



The Knock Beds form a small but exceedingly well-marked group of rocks, which 

 are typically developed near the village of Knock in Westmoreland, but are found 

 everywhere surmounting the Skelgill graptolitic beds. They consist mainly of 

 green and purple shales and grits, and have yielded two species of Graptolites, viz. 

 Monocp-aptiis priodon and M. Browjhtonensis, n. sp. The authors consider the 

 Knock Beds to be the diminutive representative of the Gala and Ilajvick Beds of 

 the south of Scotland. They conclude, further, that tlie entire Coniston Mudstone 

 series is to be regarded as belonging to the Middle Silurian period — that is, to the 

 period in which the Loiver and UjJjJer Llandovery and the Tarnnnon Shales of 

 " Siluria " were laid down. 



Finally, the authors conclude that the true Coniston Flags are to be entirely 

 separated from the Coniston Mudstone series. They parallel the Coniston Flags 

 with the Denbighshire Flags of North Wales and the Balmae and Ricrurton Beds 

 of the south of* Scotland, and they regard them as forming the true base of the 

 Upper Silurian series. 



The Cause of the Qlacial Period, with reference to the British Isles*. 

 By Chables Eickexts, M.D., F.Q.S. 



Some wlio consider that the Glacial Period was dependent on extreme cold caused 

 by the winters occm-ring when the earth was in aphelion, with a greatly increased 

 eccentricity of its orbit, have deduced inferences which do not appear to_ accord 

 with present physical conditions. With great glacier systems, such as existed m 

 North America and in Europe, the an- would have had almost the whole of its 

 moisture condensed out of it by the cold long before it reached the Arctic circle ; 

 consequently glaciers could not have existed on the water-slope of the land sur- 

 rounding the Arctic Ocean any more than they do now ; and, so far from an _" ice- 

 cap " covering the Arctic regions, there would not have been sufficient moisture 

 lett to form in it ice-floes so great or extensive as at present. 



The increased accumulation of snow now talring place in Greenland is accom- 

 panied by subsidence of the land, whilst elevation is at the same time rapidly 

 occurring in Norway and Spitsbergen; it is therefore not requisite to attribute 

 "the invariable occurrence of submergence along with glaciatiou to change in the 

 centre of gravity of the earth." The cause of the present subsidence of Greenland, 

 as well as that in Britain during tlie Glacial Period, was ascribed m the eftcct 

 which an increased weight of snow would have in forcing downwards the crust of 

 the earth into its fluid substratum (see Geol. Mag. vol. ix. p. 119). 



Previous to the Glacial Period the Gulf of Mexico extended as far north as the 



junction of the Mississippi and the Ohio, Florida was submerged, and an extensive 



belt of land on the east coast of the United States was covered by a sea hnvin^ a, 



tropical temperatm'c. Such alterations in the coast-lines must have induced a 



* Printed in extenso in the ' Geological Magazine ' for December 1875. 



